1. Field of the Invention
The present invention relates to an information recording medium on which recording data is recorded by irradiating a laser beam.
2. Description of the Related Art
An optical information recording medium disclosed by Japanese Laid-Open Patent Publication No. H09-138972 is known as this type of information recording medium. In this optical information recording medium, an intermediate layer, a light absorbing layer, a light reflective layer, and a protective layer (hereinafter, collectively referred to as “functional layers” when no distinction is required) are formed in that order by sputtering or spin coating on a substrate in which grooves have been formed with a track pitch of around 1.6 μm. Here, with this optical information recording medium, by irradiating a laser beam from the substrate side, regions of the light absorbing layer irradiated with the laser beam heat up and expand, causing partial deformation of the intermediate layer and/or the substrate. By doing so, the deformed parts in the intermediate layer and/or the substrate are recorded as recording marks, resulting in recording data being recorded.
However, by investigating the conventional optical information recording medium, the present inventors discovered the following problems. That is, in the conventional optical information recording medium, a reflective layer for reflecting the laser beam irradiated from the substrate (base member) side is formed so as to contact the light absorbing layer. Here, in the optical information recording medium, the reflective layer is formed of a metal film of gold, silver, copper, aluminum, or the like (i.e., a thin film with high thermal conductivity). Here, when the light absorbing layer heats up due to the irradiation of a laser beam during the recording of recording data, such heat is rapidly conducted to the reflective layer. Accordingly, with a conventional optical information recording medium, it is necessary to sufficiently irradiate a laser beam onto the light absorbing layer so that the temperature of the regions of the light absorbing layer that are irradiated with the laser beam rises sufficiently to cause expansion. This means that with the conventional optical information recording medium, there is the problem that if the irradiated amount of the laser beam is too low (i.e., the irradiation time is short) when recording marks are formed, there is the risk of recording errors occurring, so that the high speed recording of recording data is difficult. The conventional optical information recording medium is also constructed with a large number of functional layers that are formed by sputtering or spin coating. Accordingly, since a large number of sputtering or spin coating processes are required to form the large number of functional layers, there is a further problem in that it is difficult to reduce the manufacturing cost of the conventional optical information recording medium due to the large number of manufacturing processes.